1. Field of the Invention
The present invention relates to an interference fit type cutting tool in which a tool is detachably and replaceably mounted in a tool holding section.
This specification is based on patent applications filed in Japan (Japanese Patent Applications Nos. 10-151753 and 10-350405), and the descriptions in the Japanese applications are incorporated as a part of this specification.
2. Background Art
A boring bar as an example of this type of conventional cutting tool is shown in FIGS. 18 and 19.
This boring bar 1 is formed by detachably fitting a head section 4 having a throwaway tip 3 on a shank section 2 held by a machining center or the like (not shown). In this case, the shank section 2 is made of, for example, expensive cemented carbide having high rigidity. Since the head section 4 is prone to wear and damage due to chip abrasion, it is made of inexpensive steel or the like, and it is detachable and replaceable.
By making the head section 4 replaceable, head sections having different types of cutting edges can be mounted.
Regarding the structure of a fitting section 5 between the head section 4 and the shank section 2 in such a boring bar 1, for example, a projection 6a formed at the center of a leading end face 6 of the shank section 2 is prismatically shaped, and a hole portion 7a in a base end face 7 of the head section 4 is formed in the shape of a prismatic recess. The projection 6a is fitted into the hole portion 7a, so that the leading end face 6 and the base end face 7 are in contact with each other, and the peripheral portions thereof are brazed.
In another structure, the projection 6a is formed in a substantially cylindrical shape and is externally threaded, the hole portion 7a is shaped like a substantially cylindrical recess and is internally threaded to be screwed on the external thread of the projection 6a, and the projection 6a and the hole portion 7a are fitted together.
In a further structure, the projection 6a and the hole portion 7a are subjected to shrinkage fitting.
In such structures of the fitting section 5, however, when a rotating work material is subjected to boring by a cutting edge of the throwaway tip 3, the principal force of cutting resistance is applied in a direction nearly orthogonal to the cutting edge, and is received mainly by the projection 6a and the hole portion 7a, and therefore, a heavy load is imposed on the head section 4 made of steel having a substantially low rigidity. The corners of the prismatic hole portion 7a are apt to crack in the former case, and the screwed portion between the internal thread and the external thread is apt to be damaged in the latter case. Stiffness of the boring bar 1 is low, and tool life is short.
In the case of shrinkage fitting, when the hole portion 7a of the head section 4 is heated and fitted on the projection 6a, and then contracts due to the fall in temperature to ordinary temperature, the base end face 7 of the head section 4 sometimes separates from the leading end face 6 of the shank section 2 due to errors in shrinkage. For this reason, the holding strength and stiffness of the boring bar 1 are low, and tool life is short.